An axial hydraulic machine has been developed for many years, and performance of the axial hydraulic machine has been significantly improved. On the other hand, an operation for satisfying desired economic requirements, for example, operation at a flow rate which is different from a design point, is increased in frequency. Thus not only efficiency improvement of the axial hydraulic machine, but also long life, low vibration, or low noise of the machine are required, and improvement on cavitation performance is increasingly required. Although the improvement on the cavitation performance has been developed for many years, it is still an important development theme.
Improving the cavitation performance and securing efficiency of an axial hydraulic machine are conflicted. Specifically, to improve the cavitation performance, flowing water pressure applied on a runner vane, especially a suction surface thereof, is preferably larger than a saturated vapor pressure at every operation points within a predetermined operation range. In this manner, when a pressure on the suction surface is increased, occurrence of cavitation can be suppressed. However, due to pressure increase on the suction surface, a pressure difference between a pressure surface and the suction surface of a runner vane can be reduced. Therefore, a rotation torque of a runner is reduced since a force applied to the runner vane is reduced. As a result, the efficiency of the hydraulic machine might be reduced. As described above, it is difficult to compatibly achieve to improve cavitation performance and secure the efficiency of the hydraulic machine.